Optical glass

ABSTRACT

An optical glass has optical constants which are an refractive index (nd) of 1.70-1.75 and an Abbe number (νd) of 45.0-54.0; a glass transformation temperature (Tg) of 500-580° C. The glass has the following composition in mass percent of: SiO 2  more than 5 to 15%; B 2 O 3  20 to less than 30%; a total amount of SiO 2 +B 2 O 3  more than 25 to 40%; La 2 O 3  more than 21 to less than 30%; Y 2 O 3  more than 5 to 15%; Gd 2 O 3  0 to less than 10%; ZrO 2  1-8%; Nb 2 O 5  0.1-5%; Ta 2 O 5  more than 5 to 12%; a total amount of ZrO 2 +Nb 2 O 5 +Ta 2 O 5  7-20%; ZnO 0-10%; CaO 0-10%; SrO 0-5%; BaO 0-10%; a total amount of ZnO+CaO+SrO+BaO 5-15%; Li 2 O 1-8%; Sb 2 O 3  0-1%; and As 2 O 3  0-1%. Devitrification is not generated when the optical glass is kept at a temperature of 920° C. for two hours.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical glass of a lanthanumborosilicate system having a low glass transformation temperature (Tg)and excellent resistance to devitrification property, the optical glasshaving optical constants which are a refractive index (nd) in a range of1.70 to 1.75 and an Abbe number (νd) in a range of 45.0 to 54.0, theoptical glass being suitable for forming a glass preform material usedfor precision press molding, and being suitable for precision pressmolding.

2. Description of Related Art

Precision press molding of a glass is a technique to obtain a glassmolded product having a shape of a final product or a shape extremelyclose to the final product and surface accuracy by performing pressmolding to a glass preform material, which is soften by heating, under ahigh temperature by using a molding die having a cavity with apredetermined shape. According to the precision press molding, it ispossible to manufacture a molded product with a desired shape on thebasis of high productivity without performing grinding and polishing, orhardly performing grinding and polishing after molding. Therefore, atpresent, glass molded products, such as spherical lenses, asphericallenses and the like, have been manufactured by precision press molding.

Recently, miniaturizing and lightening of optical devices have beenremarkably progressing. Aspherical lenses are used so as to decrease thenumber of lenses that construct an optical system of the opticaldevices. Since it is extremely difficult to manufacture an asphericallens in large quantities and inexpensively by a method according togrinding and polishing in earlier technology, the above-describedprecision press molding is the most suitable molding method formanufacturing particularly an aspherical lens.

In order to obtain a glass molded product by the precision pressmolding, it is necessary to perform press molding to a glass preformmaterial under a high temperature, as described above. Therefore, themolding die used in this case is also exposed to the high temperature,and a high pressure is added thereto. Thereby, in respect to the opticalglass that constructs the glass preform material, it is desired to makeits glass transformation temperature (Tg) as low as possible from theviewpoint of preventing the wear of the molding surface of the moldingdie caused by oxidation of the surface in accordance with hightemperature environment and the damage of the molding surface of themolding die in accordance with the high press pressure when pressmolding is performed.

As a method for manufacturing a glass preform material for performingprecision press molding, there is a method for manufacturing a glasspreform material having a shape close to the shape of the lens, which isa final product, or having a spherical shape by obtaining a glass blockcut from a glass block material, and by grinding and polishing the glassblock. However, since the cutting process of the glass block material,grinding and polishing processes are required, there is a problem thatthose processes take time. Further, there is a method for obtaining aglass preform material by dropping or flowing down a molten glass fromthe tip of an efflux pipe connected to a glass melting device, formingthe molten glass in a die or the like, and cooling it. In this method,cutting from a glass block material, grinding and polishing processesare not required, and it is possible to obtain the glass preformmaterial directly from the molten glass. Therefore, as a method formanufacturing a glass preform material, the latter method is the methodthat has the highest mass-productivity and the manufacturing cost is thelowest. The shape of the glass preform material obtained by the lattermethod is a biconvex lens-like or a spherical shape. In many cases, thebiconvex lens-like shape is the shape close to the shape of anaspherical lens, which is a final product, or the like. Therefore,variations in shape when precision press molding is performed can bemade small, and the biconvex lens-like glass preform material has aneffect of remarkably improving the mass-productivity of the lens itself.Further, when it is a spherical shape, although variations in shape whenprecision press molding is performed become large in many cases, thereis a merit that the glass preform material can be set easily in thecenter of a lower die of the molding die whose molding surface isusually a concave surface.

Incidentally, in the press molding technique of glasses in earliertechnology, even though devitrification was generated on a surface of aglass preform material or a pressed glass molded product, the portion ofdevitrification on the surface was removed by grinding or polishingperformed after press molding. Therefore, it was not a problem ifdevitrification was not generated in the inside of the glass. However,in the precision press molding, a glass molded product to whichprecision pressing is performed without performing grinding andpolishing or hardly performing grinding and polishing after molding isused as an optical element, such as a lens or the like. Therefore, itcannot be used as a product even if devitrification is generated only ona surface of a glass preform material or a glass molded product. Thatis, in the glass preform material used for precision press molding andthe optical glass used for precision press molding, it is required thatdevitrification is not generated at a temperature suitable for formingthe glass preform material, and moreover, that devitrification is notgenerated when precision press molding is performed to the obtainedglass preform material, in addition to the glass transformationtemperature (Tg) being low, as mentioned above.

Devitrification of a glass is caused when the temperature range of theglass is in the range that the nucleation temperature range and thecrystal growth temperature range, which is in the high temperature siderather than the nucleation temperature range, are overlapped. The longerthe time that the glass is exposed to this temperature range is, themore the crystal is grown and the devitrification progresses. In themethod for manufacturing a glass preform material by dropping or flowingdown a molten glass, as mentioned above, when the viscosity of themolten glass for dropping or flowing down from the tip of an efflux pipeis too low, it becomes difficult to obtain a preform material having asmooth curved surface and a spherical shape or a shape close to abiconvex lens-like shape. Further, when the viscosity of the moltenglass is too high, both dropping a glass having a weight for one pieceof preform material from the tip of the efflux pipe, and separating aglass gob having a weight for one piece of preform from the molten glassflow flowed down from the tip of the efflux pipe by surface tension orthe like, become difficult. Therefore, it is desired to adjust theviscosity (η) of the molten glass for being dropped or flowed down in arange of logη=approximately 1.5 to approximately 2.5.

Incidentally, for the molten glass for being dropped or flowed down fromthe tip of the efflux pipe, it is comparatively hard to generatedevitrification since the molten glass does not remain in thetemperature range that the nucleation temperature range and the crystalgrowth temperature range are overlapped for a long time generally.However, for the glass deposited to the peripheral portion of the tip ofthe efflux pipe, devitrification is easily generated since it is exposedto the temperature range lower than the upper limit of the temperaturerange that the nucleation temperature range and the crystal growthtemperature range are overlapped, that is, the temperature range thatdevitrification is generated, for a long time. Then, the glass in whichdevitrification is generated is gradually involved in the molten glasswhich is dropped or flowed down. Therefore, after forming of the preformmaterial is started, devitrification becomes to be included in the glasspreform material as time passes.

As a method for preventing the above-mentioned devitrification, there isa method of raising the temperature of the tip of the efflux pipetemporarily to the temperature that devitrification of the glassdisappears for every predetermined time. However, as mentioned above,while the temperature of the tip of the efflux pipe is raised, theviscosity of the glass is too low, so that it is difficult to obtain apreform material having a smooth curved surface and a spherical shape ora shape close to a biconvex lens-like shape. Thereby, when thetemperature of the tip of the efflux pipe is raised frequently, theproductive efficiency deteriorates remarkably. Therefore, in order tomanufacture a glass preform material stably and continuously, it isrequired to be a glass that the upper limit of the temperature rangewhich the nucleation temperature range and the crystal growthtemperature range are overlapped is as low as possible. Moreover, it isrequired to be a glass that devitrification is not generated even thoughit is kept in the temperature suitable for forming a preform materialfor a long time.

Further, when the precision press molding is performed by heating andsoftening a glass preform material, devitrification is generated sincethe glass preform material is exposed to the temperature range higherthan the lower limit of the temperature range that the nucleationtemperature range and the crystal growth temperature range areoverlapped. Therefore, it is required to be a glass that the lower limitof the temperature range that the nucleation temperature range and thecrystal growth temperature range are overlapped is as high as possible,and that devitrification is not generated even though it is kept in thetemperature suitable for precision press molding.

The optical glass used for an aspherical lens or the like is required tohave various optical constants. In particular, a glass having opticalconstants that are a refractive index (nd) in a range of 1.70 to 1.75and an Abbe number (νd) in a range of about 45 to 54 is required. Inearlier technology, as a glass having optical constants in theabove-described ranges, a lanthanum borosilicate system glass and alanthanum borate system glass are known as typical glasses. However, theglasses of such systems have high glass transformation temperature (Tg),and moreover, devitrification is easily generated in many cases.Therefore, from the above-mentioned reasons, a glass having a low glasstransformation temperature (Tg) and excellent resistance todevitrification property is required. Further, the general standard ofthe superiority or inferiority in a light transmittance in an opticalglass is a wavelength which shows 80% of spectral transmittanceincluding a reflection loss in a short wavelength side. The shorter thewavelength is, the better the light transmittance is, so that it can besaid that it is a glass with little coloring. However, in the lanthanumborosilicate system glass and the lanthanum borate system glass thathave not less than 1.70 of refractive index (nd), the wavelength showingabove-described 80% is not less than 400 nm in many cases, so thatimprovement in light transmittance is also required.

For example, the Japanese Patent Laid-open Publication No. Sho 54-3115discloses an optical glass of B₂O₃—SiO₂—La₂O₃—ZrO₂—SnO₂-bivalent metaloxide system. Although this glass has an excellent light transmittance,its glass transformation temperature (Tg) is high, so that it isunsuitable for using in precision press molding. Further, melting andrefining of a lanthanum system optical glass is performed by using amelting device having a crucible or refining tank made of platinum orplatinum alloy. While the glass is melted, alloying of the Pt of thecrucible or the refining tank and the Sn of this glass is performed, andthe alloyed portion is inferior to in heat resistance. Therefore, theremay happen an accident such that a hole is opened in the crucible or therefining tank and the molten glass is flowed out from the hole. Thecause of this accident is considered because the SnO₂ included as anessential component in the glass is reduced to be SnO while melting orrefining is performed, and moreover to Sn. Such an accident is rarelycaused. However, when once occurred, economical loss is extremely largesince the melting must be stopped immediately so as to dispose themelted glass by flowing it out, the melting device must be dismantled,and the alloyed platinum or platinum alloy must be recast, and then thehole-opened crucible or the refining tank must be repaired. Therefore,it must be said that this glass is unsuitable for producing safely andin large quantities by using a melting device having a crucible or arefining tank or the like, a portion which contacts with the moltenglass being formed by a platinum or a platinum alloy.

Further, the Japanese Patent Laid-open Publication No. Sho 60-221338discloses an optical glass of B₂O₃—La₂O₃—Y₂O₃—Li₂O-bivalent metal oxidesystem. The object of this glass is to improve its resistance todevitrification property. However, its resistance to devitrificationproperty is not sufficient to form a glass preform material by using theabove-mentioned method of dropping or flowing down a molten glass.

Further, in earlier technology, a glass including a PbO component or aglass including a fluorine component is known as a glass having a lowglass transformation temperature (Tg). However, for the glass includingthe PbO component, since the glass is easily fused with the die whenprecision press molding is performed, it is difficult to use the dierepeatedly. Therefore, it is unsuitable for using in precision pressmolding. Further, for the glass including the fluorine component, whenforming of a preform material is performed, the fluorine component isselectively volatilized from the surface layer of the molten glass andclouds are generated in the preform material, or the fluorine componentis volatilized and deposited to the die when precision press molding isperformed to the preform material, and clouds are generated on thesurface of the die or the surface of the glass molded product to whichthe precision press molding is performed. For these reasons or the like,it is not suitable as a glass for manufacturing a glass preform materialfor using in precision press molding or as a glass for using inprecision press molding.

SUMMARY OF THE INVENTION

An object of the present invention is to improve synthetically thedefects seen in the glasses in earlier technology, and is to provide anoptical glass of a lanthanum borosilicate system suitable for forming aglass preform material for using in precision press molding and suitablefor precision press molding, the optical glass having optical constantswhich are a refractive index (nd) in a range of 1.70 to 1.75 and an Abbenumber (νd) in a range of 45.0 to 54.0, and having a low glasstransformation temperature (Tg) in a range of 500 to 580° C., and theoptical glass having excellent resistance to devitrification propertyand an excellent light transmittance.

In order to accomplish the above-described object, the inventor hasexamined and researched an optical glass. As a result, the inventor hasfound that an optical glass having optical constants in the desiredranges and a low glass transformation temperature, the optical glasshaving further excellent resistance to devitrification property whenforming of a preform material and precision press molding are performedwhile an excellent light transmittance is maintained, is obtained in alimited and particular composition range ofSiO₂—B₂O₃—La₂O₃—Y₂O₃—ZrO₂—Nb₂O₅—Ta₂O₅—Li₂O—ZnO and/or CaO and/or SrOand/or BaO system composition that is not concretely disclosed inearlier technology. Then, the present invention has been accomplished.

That is, in order to accomplish the above-described object, according toan aspect of the present invention, an optical glass comprises: opticalconstants which are a refractive index (nd) in a range of 1.70 to 1.75and an Abbe number (νd) in a range of 45.0 to 54.0; a glasstransformation temperature (Tg) in a range of 500 to 580° C.; more than5 to 15 mass % of SiO₂; 20 to less than 30 mass % of B₂O₃; a totalamount of SiO₂+B₂O₃ being more than 25 to 40 mass %; more than 21 toless than 30 mass % of La₂O₃; more than 5 to 15 mass % of Y₂O₃; 0 toless than 10 mass % of Gd₂O₃; 1 to 8 mass % of ZrO₂; 0.1 to 5 mass % ofNb₂O₅; more than 5 to 12 mass % of Ta₂O₅; a total amount ofZrO₂+Nb₂O₅+Ta₂O₅ being 7 to 20 mass %; 0 to 10 mass % of ZnO; 0 to 10mass % of CaO; 0 to 5 mass % of SrO; 0 to 10 mass % of BaO; a totalamount of ZnO+CaO+SrO+BaO being 5 to 15 mass %; 1 to 8 mass % of Li₂O; 0to 1 mass % of Sb₂O₃; and 0 to 1 mass % of As₂O₃; whereindevitrification is not generated when the optical glass is kept at atemperature of 920° C. for two hours.

Further, the devitrification may not be generated when the optical glassis kept at a temperature of the glass transformation temperature(Tg)+80° C. for 30 minutes.

Moreover, the devitrification may not be generated when the opticalglass is kept at a temperature of the glass transformation temperature(Tg)+140° C. for 30 minutes.

The optical glass according to the present invention has excellentresistance to devitrification property. However, in order to manufacturea glass preform material stably and continuously by using a method ofdropping or flowing down a molten glass from a tip of an efflux pipe, itis preferable to keep the optical glass at the temperature of 920° C.for two hours so as not to generate devitrification.

Further, the temperature of the glass preform material when precisionpress molding is performed differs according to the high or low of apress pressure. When a molding die is not worn even though the presspressure is made high, it is sufficient if it is the temperature of theglass transformation temperature (Tg)+80° C. When the press pressure ismade low, it is sufficient if it is the temperature of the glasstransformation temperature (Tg)+140° C. The time taken for the precisionpress molding itself is generally about several-tens seconds. However,considering the step of heating and softening the glass preform materialbefore precision press molding, and the step of annealing a glass moldedproduct after the precision press molding, it is desired that thedevitrification is not generated when it is kept at the above-describedtemperatures for 30 minutes, for safety. Therefore, it is preferablethat the devitrification is not generated when it is kept at thetemperature of the glass transformation temperature (Tg)+80° C. for 30minutes. Moreover, it is also preferable that the devitrification is notgenerated when it is kept at the temperature of the glass transformationtemperature (Tg)+140° C. for 30 minutes.

Furthermore, the optical glass may be substantially free of a fluorine,a PbO, a WO₃ and an SnO₂ components.

PREFERRED EMBODIMENT OF THE INVENTION

The reason why the composition range of each component is limited asdescribed above in an optical glass according to the present inventionwill be explained in the following.

More than 5% of the SiO₂ component need to be included in the opticalglass in order to improve the resistance to devitrification property ofthe glass and to maintain an excellent light transmittance. However,when its amount is over 15%, it becomes difficult to maintain the lowglass transformation temperature (Tg). Therefore, it is limited in arange of more than 5% to 15%.

The B₂O₃ component is a component added in order to improve theresistance to devitrification property of the glass and to keep theglass transformation temperature (Tg) low. When its amount is less than20%, the resistance to devitrification property of the glassdeteriorates. Further, when its amount is not less than 30%, thechemical durability of the glass becomes bad and it becomes difficult toobtain the desired optical constants. Therefore, it is limited in arange from 20 to less than 30%.

The total amount of the SiO₂ and the B₂O₃ components should be in arange of more than 25% to 40% in order to maintain the excellentresistance to devitrification property and the target optical constants.Further, in order to obtain a glass particularly excellent in the lighttransmittance, it is preferable that the total amount of the SiO₂ andthe B₂O₃ components is more than 32%.

The La₂O₃ component is an effective component for increasing therefractive index of the glass and lowering dispersion (making the Abbenumber large) in the glass. When its amount is not more than 21%, it isimpossible to make a refractive index of the glass into a desired value.Further, when its amount is not less than 30%, the resistance todevitrification property of the glass deteriorates. Therefore, it islimited in a range of more than 21% to less than 30%. Further, it ispreferable that its amount is not more than 27%, since a glass excellentin resistance to devitrification property can be obtained easily.

The Y₂O₃ component is an effective component for increasing therefractive index of the glass and lowering dispersion (making the Abbenumber large) in the glass, and is a component having an effect ofimproving the resistance to devitrification property of the glass. Whenits amount is not more than 5%, it becomes difficult to obtain thedesired resistance to devitrification property. Further, when its amountis over 15%, the resistance to devitrification property deteriorates onthe contrary. Therefore, it is limited in a range of more than 5% to15%.

The Gd₂O₃ component is an effective component for increasing therefractive index of the glass and lowering dispersion (making the Abbenumber large) in the glass. However, when its amount is not less than10%, the resistance to devitrification property deteriorates. Therefore,it is limited in a range of 0 to less than 10%.

The ZrO₂ component has an effect of adjusting the optical constants andimproving the resistance to devitrification property of the glass.However, when its amount is less than 1%, no remarkable effect can beseen. Further, when its amount is over 8%, the resistance todevitrification property deteriorates on the contrary. Therefore, it islimited in a range of 1 to 8%.

The Nb₂O₅ component has an effect of adjusting the optical constants andimproving the resistance to devitrification property of the glass.However, when its amount is less than 0.1%, no remarkable effect can beseen. Further, when its amount is over 5%, the resistance todevitrification property deteriorates on the contrary. Therefore, it islimited in a range of 0.1 to 5%.

The Ta₂O₅ component has an effect of adjusting the optical constants andimproving the resistance to devitrification property of the glass.However, when its amount is not more than 5%, no remarkable effect canbe seen. Further, when its amount is over 12%, the resistance todevitrification property deteriorates on the contrary. Therefore, it islimited in a range of more than 5% to 12%.

Further, in the present invention, it is particularly important to makethe three components of the ZrO₂, Nb₂O₅ and Ta₂O₅ coexist in order toobtain a glass excellent in both the resistance to devitrificationproperty in the glass preform material forming temperature range and theresistance to devitrification property in the precision press moldingtemperature range. When the total amount of these three components isless than 7%, no remarkable effect in respect to the improvement in theresistance to devitrification property can be seen. When the totalamount of these three components is over 20%, the devitrification iseasily generated on the contrary. Therefore, the total amount of thesethree components is limited in a range of 7 to 20%. Further, it ispreferable that the total amount of these three components is not lessthan 11.5% in order to obtain a glass particularly excellent in theresistance to devitrification property easily.

The ZnO component has an effect of lowering the glass transformationtemperature (Tg) and improving the resistance to devitrificationproperty of the glass. However, when its amount is over 10%, theresistance to devitrification property deteriorates on the contrary.Therefore, it is limited in a range of 0 to 10%.

Each of the CaO, SrO and BaO components has an effect of adjusting theoptical constants and improving the resistance to devitrificationproperty of the glass. However, when the amounts of the CaO, SrO and BaOcomponents are over 10%, 5% and 10%, respectively, the resistance todevitrification property deteriorates on the contrary, and the chemicaldurability also deteriorates.

Further, when the total amount of one or more selected from the ZnO,CaO, SrO and BaO components is less than 5%, the resistance todevitrification property of the glass is not improved sufficiently.Further, when the total amount of these components is over 15%, theresistance to devitrification property tends to deteriorate on thecontrary. Therefore, the resistance to devitrification property of theglass becomes good when the total amount of these components is in arange of 5 to 15%.

Further, it is preferable that the total amount of these components isnot more than 14% in order to obtain a glass particularly excellent inthe resistance to devitrification property.

The Li₂O component is a component having an effect of lowering the glasstransformation temperature (Tg). However, when its amount is less than1%, that effect cannot be obtained. Further, when its amount is over 8%,the resistance to devitrification property deteriorates rapidly.Therefore, it is limited in a range of 1 to 8%.

The Sb₂O₃ and As₂O₃ component can be added in order to defoam when theglass is melted. In order to obtain an effect of defoaming, it issufficient when the amounts of these components are up to 1%,respectively.

Further, besides the above-described components, a proper amount ofcomponents, such as Na₂O, K₂O, Rb₂O, Cs₂O, MgO, TiO₂, HfO₂, Al₂O₃, P₂O₅,Ga₂O₃, In₂O₃, GeO₂, TeO₂, CeO₂, Tl₂O, Bi₂O₃, Yb₂O₃ and the like, can beincluded in the optical glass according to the present invention withina range of the object of the present invention in order to adjust theoptical constants, to improve the melting property, to improve thechemical durability, or the like.

Further, as mentioned above, since the fluorine and PbO components arethe components having an influence which is not preferable when formingof a glass preform and precision press molding are performed, thosecomponents should not be included in the optical glass according to thepresent invention. Further, since the WO₃ component is a component fordeteriorating the light transmittance of the glass and for increasingthe coloration, it should not be included in the optical glass accordingto the present invention. Moreover, since the SnO₂ component is acomponent having a risk for causing a serious accident such that a holeis opened in a platinum crucible or the like while the glass is melted,it should not be included in the optical glass according to the presentinvention.

EXAMPLES

Hereinafter, preferred examples according to the present invention willbe explained. In addition, the present invention is not limited to theexamples in the following.

Compositions of the examples of the optical glasses according to thepresent invention (No. 1 to No. 10) and those of the comparativeexamples of the optical glasses in earlier technology (No. A to No. E),and the refractive index (nd), the Abbe number (νd), the glasstransformation temperature (Tg) and the coloration of the glass obtainedin each of the examples and comparative examples are shown in Table 1 toTable 3.

Here, the glass transformation temperatures (Tg) were obtained basedupon “Measuring Method for Thermal Expansion of Optical Glass”JOGIS08-¹⁹⁷⁵ Japanese Optical Glass Industrial Standards. Each sample,which is a round bar with a length of 50±5 mm and a diameter of 4±0.5mm, was heated so that the temperature rises in a predetermined rate of4° C. per minute. Then, a thermal expansion curve was obtained bymeasuring the temperature and the elongation of the sample. Thereafter,each glass transformation temperature was obtained from the thermalexpansion curve.

Further, the colorations were obtained based upon “Measuring Method forColoration of Optical Glass” JOGIS02-¹⁹⁷⁵, Japanese Optical GlassIndustrial Standards. The spectral transmittance including a reflectionloss of each sample with a thickness of 10±0.1 mm, the facing surfacesof which are polished in parallel, was measured. The first place of theinteger of the wavelength showing a transmittance of 80% was roundedoff, so that each coloration was shown as making 10 nm as a unit. Thesmaller the value of the coloration is, the more excellent the lighttransmittance in the short wavelength side is.

In addition, for the optical glasses in the examples No.1 to No.10according to the present invention, normal optical glass raw materials,such as oxides, carbonates, nitrates or the like, were weighed and mixedat the predetermined ratio so as to obtain the composition ratios inTable 1 and Table 2. Then, they were put into a 300 cc platinum crucibleand molten at a temperature of 1000 to 1300° C. for two to four hoursaccording to difficulty of the melting property depending on thecomposition of the glass and then it was stirred and homogenized.Thereafter, its temperature was lowered, and it was cast in a die or thelike and annealed. Thereby, the optical glasses in the examples No.1 toNo.10 were easily obtained.

Further, the results of devitrification tests of the glass in each ofthe above-described examples and comparative examples are shown in Table4 and Table 5. Here, for the devitrification test 1 for evaluating theresistance to devitrification property when the glass preform is formed,100 g of each glass raw material weighed and mixed for the predeterminedratio was put into a 50 cc crucible made of platinum so that thecomposition ratio of each example and each comparative example becomesas shown in Table 1 to Table 3. Then, each raw material was molten at atemperature of 1200 to 1300° C. for two hours in an electric furnaceaccording to difficulty of the melting property depending on thecomposition of the glass so as to be a complete glass melt withoutdevitrification. Thereafter, its temperature was lowered, and it waskept in the furnace at each temperature of 1000° C., 980° C., 920° C.,900° C. and 880° C. for two hours. Thereafter, it was taken out of thefurnace, and then observed by a microscope about the presence or absenceof the devitrification. As the results of the observation, a glass inwhich no devitrification is observed is indicated by a circle mark (◯),a glass in which devitrification is observed only on a surface isindicated by a triangle mark (Δ), and a glass in which devitrificationis observed inside is indicated by a cross mark (x).

Further, for the devitrification test 2 for evaluating the resistance todevitrification property when precision press molding is performed, eachsample, in which the glass of each example and each comparative exampleshown in Table 1 to Table 3 was made into a cube of 10 mm square, wasput onto a flat plate of a heat resistant ceramics. Then, each samplewas put into the electric furnace and its temperature was raised to eachtemperature of Tg+100° C., Tg+120° C. and Tg+140° C., and was kept atthese temperatures for 30 minutes. Thereafter, each sample was taken outfrom the furnace, and then observed by the microscope about the presenceor absence of the devitrification. As the results of the observation, aglass in which no devitrification is observed is indicated by a circlemark (◯), a glass in which devitrification is observed only on a surfaceis indicated by a triangle mark (Δ), and a glass in whichdevitrification is observed inside is indicated by a cross mark (x).

TABLE 1 (mass %) EXAMPLE No. 1 2 3 4 5 COMPOSITION OF GLASS SiO₂ 5.2 5.115.0 8.0 10.8 B₂O₃ 29.9 28.0 25.0 26.0 24.6 Y₂O₃ 10.0 12.0 14.4 5.9 12.0La₂O₃ 21.1 25.0 25.5 22.1 26.5 Gd₂O₃ 4.3 5.0 9.9 ZrO₂ 8.0 3.0 1.0 4.05.0 Nb₂O₅ 0.5 5.0 0.1 1.0 0.5 Ta₂O₅ 7.0 12.0 5.9 6.0 7.0 ZnO 3.0 5.0 2.05.5 SrO 2.0 5.0 BaO 5.0 3.0 3.0 10.0 5.0 Li₂O 3.0 1.8 3.0 2.0 3.0 Sb₂O₃1.0 0.1 0.1 0.1 0.1 TOTAL 100.0 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ 35.133.1 40.0 34.0 35.4 ZrO₂ + Nb₂O₅ + 15.5 20.0 7.0 11.1 12.5 Ta₂O₅ ZnO +SrO + 10.0 8.0 5.0 15.0 10.5 CaO + BaO nd 1.714 1.720 1.715 1.730 1.733νd 48.8 47.2 52.2 50.7 49.4 Tg(° C.) 560 562 578 543 571 COLORATION 3738 38 37 37

TABLE 2 (mass %) EXAMPLE No. 6 7 8 9 10 COMPOSITION OF GLASS SiO₂ 12.011.0 10.8 5.1 10.0 B₂O₃ 20.0 29.0 24.6 20.0 23.0 Y₂O₃ 5.2 15.0 10.0 10.09.0 La₂O₃ 29.9 22.0 23.5 28.0 24.0 ZrO₂ 7.0 3.0 6.0 7.0 6.0 Nb₂O₅ 0.50.9 0.5 2.0 0.5 Ta₂O₅ 11.0 8.0 9.0 5.1 10.0 ZnO 8.0 10.0 5.5 3.0 CaO 3.05.0 3.7 10.0 SrO 1.0 3.0 BaO 2.0 8.0 Li₂O 2.3 1.0 3.0 8.0 4.5 Sb₂O₃ 0.10.1 0.1 0.1 TOTAL 100.0 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ 32.0 40.035.4 25.1 33.0 ZrO₂ + Nb₂O₅ + 18.5 11.9 15.5 14.1 16.5 Ta₂O₅ ZnO + SrO +12.0 10.0 12.5 14.7 13.0 CaO + BaO nd 1.747 1.742 1.732 1.749 1.705 νd46.4 51.5 48.8 48.2 47.5 Tg(° C.) 542 550 565 504 527 COLORATION 39 3737 39 38

TABLE 3 (mass %) COMPARATIVE EXAMPLE No. A B C D E COMPOSITION OF GLASSSiO₂ 6.0 3.0 18.0 B₂O₃ 33.0 35.0 30.0 30.0 17.0 Y₂O₃ 16.0 18.0 4.0 2.07.0 La₂O₃ 22.0 22.0 16.0 40.0 25.0 ZrO₂ 4.0 7.0 5.9 6.0 Nb₂O₅ 6.0 Ta₂O₅7.8 13.0 7.0 ZnO 2.0 35.0 2.0 CaO 12.0 6.0 8.0 BaO 2.0 4.0 13.0 Li₂O 3.01.2 2.0 1.1 2.2 Al₂O₃ 0.8 WO₃ 2.0 3.0 TOTAL 100.0 100.0 100.0 100.0100.0 SiO₂ + B₂O₃ 39.0 38.0 30.0 30.0 35.0 ZrO₂ + Nb₂O₅ + 4.0 14.8 13.018.9 6.0 Ta₂O₅ ZnO + SrO + 16.0 6.0 35.0 6.0 21.0 CaO + BaO nd 1.7051.734 1.744 1.800 1.714 νd 53.5 49.7 45.4 43.4 50.3 Tg(° C.) 550 633 513619 584 COLORATION 38 38 39 41 42

TABLE 4 EXAMPLE No. 1 2 3 4 5 6 7 DEVITRIFICATION TEST 1 1000° C. ◯ ◯ ◯◯ ◯ ◯ ◯ 960° C. ◯ ◯ ◯ ◯ ◯ ◯ ◯ 920° C. ◯ ◯ ◯ ◯ ◯ ◯ ◯ 900° C. ◯ ◯ Δ Δ ◯ ◯◯ 880° C. ◯ ◯ Δ Δ ◯ ◯ ◯ DEVITRIFICATION TEST 2 Tg + 140° C. ◯ ◯ ◯ ◯ ◯ ◯◯ Tg + 120° C. ◯ ◯ ◯ ◯ ◯ ◯ ◯ Tg + 100° C. ◯ ◯ ◯ ◯ ◯ ◯ ◯ Tg + 80° C. ◯ ◯◯ ◯ ◯ ◯ ◯

TABLE 5 EXAMPLE COMPARATIVE EXAMPLE No. No. 8 9 10 A B C D EDEVITRIFICATION TEST 1 1000° C. ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ 960° C. ◯ ◯ ◯ Δ Δ Δ X ◯920° C. ◯ ◯ ◯ X X X X X 900° C. ◯ Δ ◯ X X X X X 880° C. ◯ Δ ◯ X X X X XDEVITRIFICATION TEST 2 Tg + 140° C. ◯ ◯ ◯ X X X X Δ Tg + 120° C. ◯ ◯ ◯ XX X X ◯ Tg + 100° C. ◯ ◯ ◯ Δ X X X ◯ Tg + 80° C. ◯ ◯ ◯ Δ X Δ X ◯

As shown in Table 1 to Table 3, in the glasses in the examples accordingto the present invention (No. 1 to No.10), each refractive index (nd) isin a range within 1.70 to 1.75, each Abbe number (νd) is in a rangewithin 45.0 to 54.0, and each glass in the examples No. 1 to No. 10 havea low glass transformation temperature (Tg) in a range within 500 to580° C. Further, the glasses in the examples according to the presentinvention (No. 1 to No. 10) have the coloration between 37 and 39.Therefore, it can be recognized that each light transmittance from theshort wavelength side of the visible range to the near ultraviolet rangeis equivalent or more excellent compared with the optical glasses inearlier technology in comparative examples shown in Table 3.

Further, as shown in Table 4 and Table 5, in the glasses in the examplesaccording to the present invention (No. 1 to No.10), as a result of thedevitrification test 1, devitrification is not generated at 920° C., andthe temperature that devitrification is not generated is lower than theglasses in the comparative examples. Further, in the devitrificationtest 2, devitrification is not generated at Tg+140° C., and thetemperature that devitrification is not generated is higher than theglasses in the comparative examples. The optical glasses in the examplesaccording to the present invention are remarkably excellent in theresistance to devitrification properties at both in the temperaturerange when forming of glass preform materials is performed and in thetemperature range when precision press molding is performed, comparedwith the optical glasses in earlier technology in the comparativeexamples. In particular, it can be recognized that the optical glassesin the examples according to the present invention are suitable forforming glass preform materials and for precision press molding.

As described above, the optical glass according to the present inventionis an optical glass of SiO₂—B₂O₃—La₂O₃—Y₂O₃—ZrO₂—Nb₂O₅—Ta₂O₅—Li₂O—ZnOand/or CaO and/or SrO and/or BaO system in particular composition range,the optical glass having the optical constants which are the refractiveindex (nd) in a range of 1.70 to 1.75 and the Abbe number (νd) in arange of 45.0 to 54.0. Therefore, its glass transformation temperature(Tg) is between 500° C. and 580° C., which is low, and the resistance todevitrification properties in the temperature range when forming of theglass preform material is performed and in the temperature range whenthe precision press molding is performed are excellent. Moreover, thecoloration is small, and the light transmittance is also excellent.Therefore, it is suitable and usable for forming a glass preformmaterial used for precision press molding, and for precision pressmolding.

The entire disclosure of Japanese Patent Application No. 2001-202605filed on Jul. 3, 2001 including specification, claims, drawings andsummary are incorporated herein by reference in its entirety.

What is claimed is:
 1. An optical glass consisting of: optical constantswhich are a refractive index (nd) in a range of 1.70 to 1.75 and an Abbenumber (νd) in a range of 45.0 to 54.0; a glass transformationtemperature (Tg) in a range of 500 to 580° C.; more than 5 to 15 mass %of SiO₂; 20 to less than 30 mass % of B₂O₃; a total amount of SiO₂+B₂O₃being more than 25 to 40 mass %; more than 21 to less than 30 mass % ofLa₂O₃; more than 5 to 15 mass % of Y₂O₃; 0 to less than 10 mass % ofGd₂O₃; 1 to 8 mass % of ZrO₂; 0.1 to 5 mass % of Nb₂O₅; more than 5 to12 mass % of Ta₂O₅; a total amount of ZrO₂+Nb₂O₅+Ta₂O₅ being 7 to 20mass %; 0 to 10 mass % of ZnO; 0 to 10 mass % of CaO; 0 to 5 mass % ofSrO; 0 to 10 mass % of BaO; a total amount of ZnO+CaO+SrO+BaO being 5 to15 mass %; 1 to 8 mass % of Li₂O; 0 to 1 mass % of Sb₂O₃; and 0 to 1mass % of As₂O₃; wherein devitrification is not generated when theoptical glass is kept at a temperature of 920° C. for two hours.
 2. Theoptical glass as claimed in claim 1, wherein the devitrification is notgenerated when the optical glass is kept at a temperature of the glasstransformation temperature (Tg)+80° C. for 30 minutes.
 3. The opticalglass as claimed in claim 1, wherein the devitrification is notgenerated when the optical glass is kept at a temperature of the glasstransformation temperature (Tg)+140° C. for 30 minutes.